1. Field of the Invention
The invention relates to a process for the production of highly resilient, cold-curing polyurethane foams. In particular, it relates to foams formed from at least difunctional polyisocyanates, polyols with at least two hydroxyl groups per molecule of which at least 40% on the average are primary hydroxyl groups, the polyols having an equivalent weight per hydroxyl groups of 700 to 3,000, as well as catalysts, blowing agents, emulsifiers, stabilizers and, if necessary, other conventional additives.
2. Description of the Prior Art
In the production of so-called highly resilient polyurethane foams, a polyisocyanate which is at least difunctional, for example, toluene diisocyanate or diphenylmethane diisocyanate, is reacted with a polyol, which has at least two hydroxyl groups per molecule and which, on the average, has a high proportion of primary hydroxyl groups. Such polyols are synthesized, as a rule, by first of all adding propylene oxide to a starter alcohol and then adding ethylene oxide to this product in amounts, such that at least 40% of the hydroxyl groups, and preferably 70 to 90% of the hydroxyl groups, are present in the form of primary hydroxyl groups.
Due to the high content of primary hydroxyl groups, the polyols have a high reactivity towards isocyanates. In contrast to conventional polyurethane foams, i.e., the so-called hot foams, a high crosslinking density is therefore achieved during foaming. This has the advantage that there is no need to supply external energy during curing and that the overall time required for curing the foams is reduced. It is, however, a disadvantage that the tendency to form closed-cell foams is increased and the processing latitude is restricted. The expression "processing latitude" is understood to be the tolerance limits within which it is possible to deviate from a formulation without endangering the formation of stable and, at the same time, open-celled foams.
The processing latitude which is narrower due to the high reactivity of the foaming components, which make it more difficult to form a stable, yet open-celled foam, do not permit those products which have been successfully employed in the production of so-called hot foams, to be used as foam stabilizers.
In the production of highly resilient, cold-curing polyurethane foams, it is in principle possible, as a result of the high reactivity of the foaming raw material, to obtain stable foams without the addition of foam stabilizers, by using higher functional isocyanates or polyethers, as well as low molecular, polyfunctional crosslinking agents. However, the foams which are obtained in this manner, have a coarser cell structure and are significantly closed-celled and therefore not usable industrially.
For the purpose of regulating the cell structure of these foams, it is possible to use low molecular methyl or phenylmethylpolysiloxanes, such as those described in German Pat. No. 25 33 074 and German Offenlegungsschrift No. 22 21 811. By so doing, sufficiently open-celled foams with a controlled cell structure are obtained in a narrow domain. However, the processing latitude is narrow and, what is even more important, the physical properties of these foams are not adequate for many applications. As a consequence of their high degree of crosslinking, these foams have low values for the elongation at rupture and resistance to tearing and, moreover, a relatively low hardness.
In order to eliminate these disadvantages, highly resilient foams have been developed which are prepared with reactive polyols, predominantly difunctional isocyanates, such as, pure toluene diisocyanate (TDI) or mixtures of TDI with 20% or less of diphenylmethane diisocyanate and slight amounts of crosslinking compounds as raw materials. Besides the polyols consisting of propylene and ethylene oxides, it is possible to use polyols which contain chemically bound or physically dispersed polymeric components, such as, for example, polymers of acrylonitrile and styrene or polymeric urea derivatives, in order to improve the hardness of these foams.
Formulations on this basis do not produce inherently stable foams. Thus, unless stabilizers are added, the foams collapse once again after they have risen. The stabilizers required for these foams must therefore have a stabilizing action against relapse, as well as a cell regulating action and must ensure the formation of open-celled foams over as wide a range as possible.
Compounds have already been proposed to meet these requirements. The stabilizers of the state of the art can be divided into two groups:
One group is formed by polysiloxane-polyoxyalkylene copolymers, in which the polysiloxane blocks have a molecular weight of about 150 to 2,500 and the polyoxyalkylene blocks a molecular weight of about 150 to 1,500. The products are free of hydroxyl groups. Such products and their use in polyurethane foaming are described in U.S. Pat. Nos. 3,741,917 and 4,031,044.
The other group of stabilizers comprises polysiloxanes which are modified with organic groups. Such groups are the cyanoalkyl group (U.S. Pat. No. 3,952,038), the cyanoalkoxyalkyl group (German Auslegeschrift No. 2 402 690, the sulfolanyloxyalkyl group (U.S. Pat. No. 4,110,272), the morpholinoalkoxyalkyl group (U.S. Pat. No. 4,067,828) and the tertiary hydroxyalkyl group (U.S. Pat. No. 4,039,490).
A disadvantage of the aforementioned and, in principle, usable stabilizers is their relatively narrow processing latitude. This forces the processor to adhere to very close tolerances in metering out the foaming components, which cannot always be done with the required reliability. Besides the principal requirements which a stabilizer must meet, that is stabilization against relapse, cell regulation and cell openness after rising, the main task of the stabilizer is to exert an equalizing function over the changes which occur in practice. It must be possible to adjust foam formulations of different reactivity and stability to the desired stabilizing level by changing the concentration of the stabilizer. For this reason, the processing latitude of a stabilizer with respect to changes in the concentration is of great practical importance. Higher and lower concentrations of a good stabilizer must stabilize the foam, as well as produce foams of comparable cell openness and cell structures.
It is evident from these requirements for a foam stabilizer that these products at times have to fulfill contradictory tasks. Particularly, on the one hand, the foam must be stabilized against sagging and, on the other, the opening of the cells after the foam has reached maximum height, should be as complete as possible. In view of the complex processes that take place when different polyurethane formulations are foamed, it is hardly possible to predict the structure and effectiveness of foam stabilizers and particularly, to apply the knowledge gained, for example, in the field of conventional hot foams, to other foam systems. It is therefore essential for each foam technology to empirically test possible variations in the structure of the stabilizer molecule. The discovery of a special variation can therefore represent a significant practical improvement.